The cabin of a modern passenger aircraft is air conditioned usually both when the aircraft is flying and is on the ground by means of the aircraft's own air conditioning system. The air conditioning units of the aircraft air conditioning system, which are arranged, for example, in the wing roots of the aircraft, are supplied with bleed air which is taken from the engine compressors or auxiliary power unit compressors and cooled to a desired low temperature in the air conditioning units. If necessary, the air conditioning units can also deliver warm air for heating the aircraft cabin. The air cooled in the air conditioning units of the aircraft air conditioning system is led into a mixer where it is mixed with recirculation air sucked from the aircraft cabin. The mixed air produced in the mixer and composed of cold fresh air provided by the air conditioning units and of recirculation air sucked from the aircraft cabin is finally led into the aircraft cabin for air conditioning of the aircraft cabin.
In wide-body aircraft, in particular in wide-body aircraft having two passenger decks extending over the entire length of the aircraft, an air conditioning system described, for example, in DE 44 25 871 C1 and comprising two recirculation systems for sucking recirculation air from the aircraft cabin is used at present. A low-pressure recirculation system extracts air from an upper deck region of the cabin, while a high-pressure recirculation system serves to extract air from a middle deck region of the cabin. The recirculation air removed from the middle deck region of the cabin by the high-pressure recirculation system is blown into a central mixer of the aircraft air conditioning system. In contrast, the air sucked from the upper deck region by the low-pressure recirculation system is supplied to local mixers which are fed with premixed air by the central mixer, i.e. an air mixture composed of cold fresh air provided by the air conditioning units and recirculation air from the middle deck region of the cabin. The air mixture, produced in the local mixers, composed of premixed air from the central mixer and recirculation air from the upper deck region of the cabin is finally used for air conditioning the aircraft cabin. In particular, air is led into the middle deck region of the cabin from a local mixer arranged in the region of the middle deck, while air is blown into the upper deck region of the cabin from a local mixer arranged in the region of the upper deck.
In modern passenger aircraft, the design of the aircraft's own air conditioning system is usually adapted to the application-specific cooling requirements. Consequently, in the event of an increased cooling requirement, the cooling capacity of the aircraft air conditioning system must also be correspondingly increased. An increase of the cooling capacity may be achieved by enlarging the air conditioning units. However, enlarged and thus higher-capacity air conditioning units result in additional costs, additional weight and problems with the integration into the limited installation space on board the aircraft. Moreover, since the capacity of the air conditioning units is substantially dependent on the temperature and pressure of the engine bleed air supplied to the air conditioning units, the capacity of the auxiliary power unit compressor, which ensures the supply of the air conditioning units with process air when the aircraft is on the ground, must also be increased. However, an increase of the capacity of the auxiliary power unit compressor results, in turn, in additional costs, additional weight and integration problems, or at least a marked reduction of the service life of the compressor. Finally, an enlargement of the air conditioning units may possibly necessitate adaptations to further aircraft components and systems, such as, for example, the recirculation fans and the duct systems, with the result that additional costs, additional weight and integration problems may arise once again.